A typical characteristic of a conventional NAND flash channel is a time-variant function. Age, retention, and geometric location (pages on the edges of a block) of a page can make a cell voltage distribution for one page much different than a cell voltage distribution of another page. The difference is viewed as a time-variant nature. The variations make it difficult for the detector to set optimal read thresholds for every individual page. This has been the major reason of read failures. Therefore, when a hard decision read fails, a conventional strategy is to start a retry read by varying the read thresholds. The intention of such a retry is to hit the sweet spot of the voltage distributions. This kind retry is pretty much both blind (trial and error) and time consuming. At the end of the life of a solid state drive (SSD), throughput performance is expected to become slower due to an increase in retry activities.
It would be desirable to implement adaptive architecture of a channel detector for NAND flash channels.